A sexual act involving a very large woman. A "mexican rugburn" is performed by having the fat chick get naked (perferrably after foreplay so she is wet and sweaty) and then having her spread her cheeks, sit on your face, and rub her butt back and forth very rapidly. When done, you will have a rugburn on your face from her butt pubes and you will be brown like a Mexican cause of the dinkelberries.

Some fat cow gave me a Mexican Rugburn, I still can't smell anything but feces and wet fish